make default_collate_fn visible. test=develop (#25244)

* make default_collate_fn visible. test=develop

python/paddle/fluid/dataloader/dataloader_iter.py

@@ -38,7 +38,27 @@ from ..multiprocess_utils import CleanupFuncRegistrar, _cleanup_mmap, _set_SIGCH
 MP_INDICES_CHECK_INTERVAL = 5
 
 
-def _default_collate_fn(batch):
+def default_collate_fn(batch):
+    """
+    Default batch collating function for :code:`fluid.io.DataLoader`,
+    batch should be a list of samples, and each sample should be a list
+    of fields as follows:
+    
+    [[filed1, filed2, ...], [filed1, filed2, ...], ...]
+    
+    This default collate function zipped each filed together and stack
+    each filed as the batch field as follows:
+
+    [batch_filed1, batch_filed2, ...]
+
+    Args:  
+        batch(list of list of numpy array): the batch data, each fields
+              should be a numpy array, each sample should be a list of
+              fileds, and batch should be a list of sample.
+    
+    Returns:
+        a list of numpy array: collated batch
+    """
     sample = batch[0]
     # dataset has only 1 field
     if isinstance(sample, np.ndarray):
@@ -82,7 +102,7 @@ class _DataLoaderIterBase(object):
         self._return_list = loader.return_list
         self._batch_sampler = loader.batch_sampler
         self._sampler_iter = iter(loader.batch_sampler)
-        self._collate_fn = loader.collate_fn or _default_collate_fn
+        self._collate_fn = loader.collate_fn or default_collate_fn
         self._num_workers = loader.num_workers
         self._use_buffer_reader = loader.use_buffer_reader
         self._use_shared_memory = loader.use_shared_memory

python/paddle/fluid/reader.py

@@ -23,7 +23,7 @@ from .executor import global_scope
 from .data_feeder import DataFeeder, BatchedTensorProvider
 from .multiprocess_utils import multiprocess_queue_set, CleanupFuncRegistrar, _cleanup_mmap, _cleanup, _set_SIGCHLD_handler
 from .dataloader import BatchSampler, Dataset
-from .dataloader.dataloader_iter import _DataLoaderIterSingleProcess, _DataLoaderIterMultiProcess
+from .dataloader.dataloader_iter import _DataLoaderIterSingleProcess, _DataLoaderIterMultiProcess, default_collate_fn
 from .layers.io import monkey_patch_reader_methods, _copy_reader_var_, double_buffer
 from .unique_name import UniqueNameGenerator
 import logging
@@ -44,7 +44,7 @@ else:
 # NOTE: [ avoid hanging & failed quickly ] These value is used in getting data from another process
 QUEUE_GET_TIMEOUT = 60
 
-__all__ = ['PyReader', 'DataLoader']
+__all__ = ['PyReader', 'DataLoader', 'default_collate_fn']
 
 data_loader_unique_name_generator = UniqueNameGenerator()